regexEvaluater may be of interest if you would like to take a more interactive approach.

It has menu options to insert special characters, character classes, posix character classes, quantifiers and groups. There are menu options to add regex options e.g. add 'giosm' to the end of a function. You can load a text file and run the regex on it, and it will hilight the matches, there is also an option to save the transformed text. There's also an option to save the regex as a script.

Teach him the basics (., +, *, (/), character classes and the like) and what to use regexes for

Use the regex evaluator together to gain proficiency

Talk about basic pitfalls

Then, after a while we'll talk again about the traps he fell into :). By that time, I will also be able to see whether the more advanced concepts (look-ahead etc.) are really necessary for the work he needs to do.